Three-phase isolation AC / DC converter and control method

Abstract

The invention provides a three-phase isolation AC / DC converter and a control method, and relates to the technical field of switching power supplies. The three-phase isolation AC / DC converter comprises a composite compensation unit which is used for converting a three-phase alternating current output by a three-phase alternating current power supply into a direct current and controlling an error between an average value of a first current and a first reference current to be smaller than a first preset threshold value; the first current is the current corresponding to the minimum absolute value of the current in the three-phase alternating current; the isolation conversion unit is connected with the composite compensation unit and used for controlling the error between the average value of the input current of the isolation conversion unit and the second reference current to be smaller than a second preset threshold value; the first reference current and the second reference current are determined based on the target output power and the preset power factor. Through cooperative control of the composite compensation unit and the isolation conversion unit, conversion of three-phase alternating current and direct current is achieved, front-stage and back-stage decoupling is achieved without depending on a bus capacitor, therefore, a pre-charging circuit does not need to pre-charge the bus capacitor, and the starting response speed is increased.

Description

Technical Field

[0001] This invention relates to the field of switching power supply technology, and in particular to a three-phase isolated AC / DC converter and its control method. Background Technology

[0002] With the development of new energy technologies, AC / DC converters have been widely used in many fields such as data centers, intelligent computing center power supply, electric vehicle charging, and energy storage.

[0003] In related technologies, AC / DC converters typically employ a two-stage design. Taking a new energy vehicle charging system as an example, the corresponding two-stage architecture can be found in [reference needed]. Figure 1 As shown. The preceding stage is a power factor correction (PFC) circuit composed of a Vienna or full-bridge PWM rectifier; its specific architecture can be found in [reference needed]. Figure 2 As shown. The subsequent stage is an isolated DC / DC converter circuit. In the power range of tens of kilowatts to hundreds of kilowatts, the isolated DC / DC converter circuit can be a phase-shifted full-bridge or LLC. A large capacitor is usually needed on the DC bus between the front and rear stages to achieve decoupling. In addition, to avoid large surge voltages at the moment of AC power-on, an input pre-charge circuit is usually added to the input section. Specifically, a relay disconnects the three-phase mains power, a bypass resistor charges the bus capacitor, and the relay is closed once the voltage of the bus capacitor reaches the desired value.

[0004] Because the relevant technology requires pre-charging the bus capacitor through the input pre-charging circuit, the power-on response speed is relatively low. Summary of the Invention

[0005] This invention provides a three-phase isolated AC / DC converter and control method to solve the problem in related technologies where AC / DC converters need to precharge the bus capacitor through an input precharge circuit, resulting in a low start-up response speed.

[0006] In a first aspect, embodiments of the present invention provide a three-phase isolated AC / DC converter, comprising: The composite compensation unit is used to convert the three-phase AC power output from the three-phase AC power supply into DC power, and to control the error between the average value of the first current and the first reference current to be less than a first preset threshold; wherein, the first current is the current corresponding to the smallest absolute value of the current in the three-phase AC power. An isolation conversion unit, connected to a composite compensation unit, is used to control the error between the average value of the input current of the isolation conversion unit and the second reference current to be less than a second preset threshold. The first reference current and the second reference current are determined based on the target output power and the preset power factor.

[0007] In one possible implementation, the composite compensation unit includes: a first inductor, a second inductor, a third inductor, and a three-phase bridge arm; Among them, the three-phase bridge arm includes the first bridge arm, the second bridge arm and the third bridge arm connected in parallel; The first end of the first inductor is connected to the first output terminal of the three-phase AC power supply, and the second end of the first inductor is connected to the midpoint of the first bridge arm. The first end of the second inductor is connected to the second output terminal of the three-phase AC power supply, and the second end of the second inductor is connected to the midpoint of the second bridge arm. The first end of the third inductor is connected to the third output terminal of the three-phase AC power supply, and the second end of the third inductor is connected to the midpoint of the third bridge arm. The two ends of the first, second, and third bridge arms connected in parallel are respectively connected to the isolation conversion unit.

[0008] In one possible implementation, the first bridge arm includes a first switch and a second switch connected in series, the second bridge arm includes a third switch and a fourth switch connected in series, and the third bridge arm includes a fifth switch and a sixth switch connected in series. The power frequency cycle of the three-phase AC power supply includes 6 sectors. For any sector, two target switching transistors in the three-phase bridge arm are in a high-frequency switching state so that the error between the average value of the first current and the first reference current is less than the first preset threshold. The target switching transistor is one of the two switching transistors in the three-phase bridge arm connected to the target inductor. The target inductor is one of the first inductor, the second inductor, and the third inductor, and the current flowing through the target inductor is the first current.

[0009] In one possible implementation, the three-phase isolated AC / DC converter further includes a first control unit, which is used to determine the error between the first current and the first reference current, and to determine the duty cycle of the target switch based on the error between the first current and the first reference current, so that the error between the average value of the first current and the first reference current is less than a first preset threshold.

[0010] In one possible implementation, the isolation conversion unit includes a first bridge circuit, a first capacitor, a fourth inductor, a transformer, a second bridge circuit, and a second capacitor. The first bridge circuit is used to convert the DC power output from the composite compensation unit into AC power, and supply it to the primary winding of the transformer through the first capacitor and the fourth inductor; wherein, the first bridge circuit includes a fourth bridge arm and a fifth bridge arm connected in parallel, the midpoint of the fourth bridge arm is connected to the first terminal of the first capacitor, the second terminal of the first capacitor is connected to the first terminal of the fourth inductor, the second terminal of the fourth inductor is connected to the first terminal of the primary winding of the transformer; the midpoint of the fifth bridge arm is connected to the second terminal of the primary winding of the transformer; The second bridge circuit is used to receive the AC power output from the secondary winding of the transformer and convert it into DC power to supply the second capacitor and the load. The second bridge circuit includes a sixth bridge arm and a seventh bridge arm connected in parallel. The first end of the secondary winding of the transformer is connected to the midpoint of the sixth bridge arm, and the second end of the secondary winding of the transformer is connected to the midpoint of the seventh bridge arm. The second capacitor is connected in parallel with both the second bridge circuit and the load for filtering.

[0011] In one possible implementation, the three-phase isolated AC / DC converter further includes a second control unit, which is used to determine the error between the input current of the isolated converter and the second reference current, and to determine the control parameters of the isolated converter based on the error between the input current of the isolated converter and the second reference current, so that the error between the average value of the input current of the isolated converter and the second reference current is less than a second preset threshold. The control parameters include at least one of the following: the duty cycle, switching frequency, and phase angle of the isolation converter unit.

[0012] In one possible implementation, the three-phase isolated AC / DC converter also includes a low-pass filter unit, which is located between the output of the three-phase AC power supply and the composite compensation unit, to filter out the high-frequency ripple current generated by the composite compensation unit and the isolation conversion unit.

[0013] In one possible implementation, the low-pass filter unit includes a fifth inductor, a sixth inductor, a seventh inductor, a third capacitor, a fourth capacitor, and a fifth capacitor; Among them, the first end of the fifth inductor is connected to the first output terminal of the three-phase AC power supply, and the second end is connected to the first end of the first inductor; the first end of the sixth inductor is connected to the second output terminal of the three-phase AC power supply, and the second end is connected to the first end of the second inductor; the first end of the seventh inductor is connected to the third output terminal of the three-phase AC power supply, and the second end is connected to the first end of the third inductor. The first terminal of the third capacitor is connected between the second terminal of the fifth inductor and the first terminal of the first inductor. The first terminal of the fourth capacitor is connected between the second terminal of the sixth inductor and the first terminal of the second inductor. The first terminal of the fifth capacitor is connected between the second terminal of the seventh inductor and the first terminal of the third inductor. The second terminals of the third capacitor, the fourth capacitor, and the fifth capacitor are interconnected.

[0014] In a second aspect, embodiments of the present invention provide a control method apparatus for a three-phase isolated AC / DC converter, applied to any three-phase isolated AC / DC converter according to any one of the first aspects, comprising: Obtain the target output power and the voltage of the three-phase AC power output from the three-phase AC power supply; The first reference current and the second reference current are determined based on the voltage of the three-phase AC power, the target output power, and the preset power factor. The composite compensation unit controls the first reference current to convert the three-phase AC power output from the three-phase AC power supply into DC power, and makes the error between the average value of the first current and the first reference current less than the first preset threshold; wherein, the first current is the current corresponding to the smallest absolute value of the current in the three-phase AC power. The isolation conversion unit is controlled according to the second reference current so that the error between the average value of the input current of the isolation conversion unit and the second reference current is less than the second preset threshold.

[0015] In one possible implementation, the first reference current and the second reference current are determined based on the voltage of the three-phase alternating current, the target output power, and a preset power factor, including: Based on the current voltage of the three-phase AC power supply, determine the sector corresponding to the three-phase AC power supply at the current moment; The target input current for the three phases is determined based on the voltage of the three-phase AC power, the target output power, and the preset power factor. Based on the three-phase target input current, determine the first reference current and the second reference current corresponding to the current sector.

[0016] In this embodiment of the invention, the three-phase isolated AC / DC converter includes a composite compensation unit and an isolated conversion unit. The composite compensation unit converts the three-phase AC power output from the three-phase AC power supply into DC power and controls the error between the average value of a first current and a first reference current to be less than a first preset threshold; wherein the first current is the current corresponding to the smallest absolute value of the current in the three-phase AC power. The isolated conversion unit is used to control the error between the average value of its input current and a second reference current to be less than a second preset threshold, thereby indirectly controlling the current waveforms of the two phases with larger absolute current values ​​in the three-phase AC power, and thus controlling the output voltage and output power. This embodiment of the application achieves the conversion of three-phase AC power to DC power through the coordinated control of the composite compensation unit and the isolated conversion unit, without relying on the bus capacitor for front-to-back decoupling. Therefore, there is no need to design a pre-charging circuit to pre-charge the bus capacitor, effectively improving the start-up response speed, and also simplifying the architecture of the three-phase isolated AC / DC converter and reducing costs. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of a two-stage AC / DC converter architecture. Figure 2 This is a schematic diagram of the circuit structure of a pre-stage Vienna rectifier; Figure 3 This is a schematic diagram of an AC / DC converter that is a matrix converter. Figure 4This is a schematic diagram of the structure of a three-phase isolated AC / DC converter provided in an embodiment of the present invention. Figure 1 ; Figure 5 This is a schematic diagram of the current, voltage, and corresponding sectors of a three-phase alternating current provided by an embodiment of the present invention; Figure 6 This is a schematic diagram of the structure of a three-phase isolated AC / DC converter provided in an embodiment of the present invention. Figure 2 ; Figure 7 This is a schematic diagram of the structure of a first control unit provided in an embodiment of the present invention; Figure 8a This is a schematic diagram comparing a first current and a first reference current provided in an embodiment of the present invention; Figure 8b This is a waveform diagram of the drive signal of the switching transistor included in a composite compensation unit provided in an embodiment of the present invention; Figure 9a This is a schematic diagram showing the comparison between the first current and the first reference current within sector 1, provided by an embodiment of the present invention. Figure 9b This is a waveform diagram of the drive signal of the switching transistor included in the composite compensation unit within sector 1, provided by an embodiment of the present invention. Figure 10 This is a schematic diagram of a composite compensation unit with a T-type three-level structure provided in an embodiment of the present invention; Figure 11 This is a schematic diagram of the structure of a second control unit provided in an embodiment of the present invention; Figure 12 This is a schematic diagram of the input current and output voltage of an isolation conversion unit provided in an embodiment of the present invention; Figure 13 This is a schematic diagram of a three-phase isolated AC / DC converter with multiphase interleaved flyback as the isolation conversion unit provided in an embodiment of the present invention; Figure 14 This is a schematic diagram of a three-phase isolated AC / DC converter with LLC as the isolation conversion unit provided in an embodiment of the present invention; Figure 15 This is a schematic diagram of the structure of a bidirectional AC / DC converter with DAB as the isolation conversion unit provided in an embodiment of the present invention; Figure 16 This is a schematic diagram of the structure of a three-phase isolated AC / DC converter provided in an embodiment of the present invention. Figure 3 ; Figure 17 A schematic diagram comparing the current waveforms of the three-phase AC power at the input and output ports of a low-pass filter unit provided in an embodiment of this application; Figure 18 This is a flowchart illustrating a control method for a three-phase isolated AC / DC converter provided in an embodiment of the present invention. Detailed Implementation

[0018] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0019] In recent years, AC / DC conversion technology with input-output isolation has developed rapidly. With the support of new devices and processes, the efficiency of AC / DC converters using two-stage schemes has gradually improved, and many single-stage AC / DC conversion technologies have also emerged.

[0020] In one related technology, a two-stage AC / DC converter typically requires a large capacitor on the DC bus between the front and rear stages to achieve decoupling. To avoid large surge voltages at the moment of AC power-on, an input pre-charge circuit is usually added to the input section to pre-charge the bus capacitor, resulting in a slower power-on response speed.

[0021] Furthermore, because the front-end stage in a two-stage design cannot achieve soft switching, even with the use of third-generation wide-bandgap semiconductors, efficiency drops significantly with increasing frequency. Currently, the switching frequency of most three-phase isolated AC / DC converters using SiC devices is below 50kHz to ensure that switching losses remain within acceptable limits. This severely restricts the continued progress in power density and cost per watt of power converters.

[0022] In another related technology, the AC / DC converter can adopt a matrix converter-type scheme, which can be found in the following reference. Figure 3 As shown, its principle is to achieve the current distribution relationship between the three-phase current and the high-frequency DC / DC link through six sets of bidirectional switches, taking into account both the three-phase power factor and output voltage regulation, while also achieving electrical isolation. Its advantages are a simple circuit, fewer power components in the entire converter, and a theoretically high maximum efficiency. However, all power semiconductors operate at high frequencies, the soft-switching logic is complex, and all bridge arms cannot be short-circuited or open-circuited, making it difficult to guarantee reliability; in addition, all power semiconductors must be high-voltage, third-generation wide-bandgap semiconductor devices, resulting in higher costs.

[0023] Based on this, this application provides a three-phase isolated AC / DC converter, including a composite compensation unit and an isolation conversion unit. The composite compensation unit converts three-phase AC power into DC power and partially shapes the three-phase AC current. The isolation conversion unit shapes the remaining portion of the three-phase AC current and controls the output voltage, output current, and output power. Specifically, the composite compensation unit converts the three-phase AC power output from the three-phase AC power supply into DC power and controls the error between the average value of the first current and the first reference current to be less than a first preset threshold; wherein, the first current is the current corresponding to the smallest absolute value of the current in the three-phase AC power. The isolation conversion unit is used to control the error between the average value of its input current and the second reference current to be less than a second preset threshold, thereby indirectly controlling the current waveform of the two phases with larger absolute values ​​of current in the three-phase AC power, and thus controlling the output voltage and output power. This embodiment achieves the conversion of three-phase AC to DC through the coordinated control of the composite compensation unit and the isolation conversion unit. It does not rely on the bus capacitor to achieve decoupling between the front and rear stages, so there is no need to design a pre-charging circuit to pre-charge the bus capacitor, which effectively improves the start-up response speed. At the same time, it simplifies the architecture of the three-phase isolated AC / DC converter and reduces the cost.

[0024] Figure 4 This is a schematic diagram of the structure of a three-phase isolated AC / DC converter provided in an embodiment of the present invention. Figure 1 ,refer to Figure 4 As shown, the three-phase isolated AC / DC converter includes: The composite compensation unit 10 is used to convert the three-phase AC power output from the three-phase AC power supply into DC power, and to control the error between the average value of the first current and the first reference current to be less than a first preset threshold; wherein, the first current is the current corresponding to the smallest absolute value of the current in the three-phase AC power. The isolation conversion unit 20 is connected to the composite compensation unit 10 and is used to control the error between the average value of the input current of the isolation conversion unit 20 and the second reference current to be less than a second preset threshold. The first reference current and the second reference current are determined based on the target output power and the preset power factor.

[0025] In one implementation scenario, the three-phase AC power output from the three-phase AC power supply is three-phase AC mains power. Figure 5 This is a schematic diagram of the current, voltage, and corresponding sectors of a three-phase alternating current provided by an embodiment of the present invention. Grid_Ia, Grid_Ib, and Grid_Ic represent the current of the three-phase alternating current, Grid_Va, Grid_Vb, and Grid_Vc represent the voltage of the three-phase alternating current, and Grid_Sector represents the sector. Figure 5 It can be seen that the voltage and current of three-phase alternating current change periodically with time.

[0026] In one implementation scenario, the power frequency cycle of a three-phase AC power supply can be divided into 6 sectors, as follows: Sector 1: Va>Vb>Vc; Sector 2: Vb>Va>Vc; Sector 3: Vb>Vc>Va; Sector 4: Vc > Vb > Va; Sector 5: Vc > Va > Vb; Sector 6: Va>Vc>Vb; Taking sector 1 as an example, at this point Va>Vb>Vc, combined with... Figure 5 As shown, the absolute value of the phase b current is the smallest at this time. Therefore, within sector 1, the phase b current is the current of the phase with the smallest absolute value of current in the three-phase AC current, that is, the phase b current is the first current. Similarly, the first current corresponding to sector 2 is the phase a current, and the first current corresponding to sector 3 is the phase c current, which will not be elaborated further here.

[0027] In some embodiments, the composite compensation unit 10 includes multiple switching transistors. By controlling the on / off state of the multiple switching transistors, synchronous rectification of the three-phase AC power output from the three-phase AC power supply and waveform control of the first current are achieved, so that the error between the average value of the first current and the first reference current is less than a first preset threshold. The average value of the first current is the average value of the first current during the high-frequency operating cycle of the composite compensation unit 10.

[0028] Optionally, the composite compensation unit 10 can be a hardware structure of a two-level full-bridge PWM rectifier, a type I three-level structure, a type T three-level structure, a five-level structure, a seven-level structure, etc.

[0029] In one implementation scenario, the voltage output by the composite compensation unit 10, i.e., the voltage between x and z, is not a basically constant DC, but rather a pulsating DC similar to the output of the three-phase uncontrolled rectifier. For details, please refer to [reference needed]. Figure 4 As shown.

[0030] The isolation conversion unit 20 is connected to the composite compensation unit 10, wherein the input current of the isolation conversion unit 20 is the DC current output by the composite compensation unit 10.

[0031] Here, within any sector, the isolation converter 20 controls the error between the average value of its input current and the second reference current to be less than a second preset threshold, thereby achieving waveform control of the currents of the two phases with larger absolute values ​​in the three-phase AC power. This process repeats cyclically across six sectors, still referencing... Figure 5As shown, this allows the three-phase AC current to be essentially consistent with the three-phase target input current throughout the entire power frequency cycle, thereby enabling control of the output voltage, output current, and overall transmission power. Furthermore, the isolation conversion unit 20 can also achieve isolated energy transmission.

[0032] Here, the average value of the input current of the isolation converter 20 is the average value of the input current during the high-frequency operating cycle of the isolation converter 20.

[0033] Optionally, the isolation converter 20 can be an isolated DC / DC converter topology such as a phase-shifted full-bridge topology with a secondary-side switch, a flyback converter, a phase-shifted full-bridge, or an LLC converter.

[0034] In this embodiment, the first reference current and the second reference current are determined based on the target output power and a preset power factor. In one implementation scenario, the sector corresponding to the current three-phase AC power supply can be determined based on the voltage of the three-phase AC power supply. The target three-phase input current can be determined based on the voltage of the three-phase AC power supply, the target output power, and the preset power factor. Based on the target three-phase input current, the first reference current and the second reference current corresponding to the current sector can be determined.

[0035] Since the composite compensation unit 10 is used to control the error between the average value of the first current and the first reference current to be less than the first preset threshold, and the first current is the current corresponding to the smallest absolute value of the current in the three-phase AC current, the target input current corresponding to the smallest absolute value of the current in the three-phase AC current can be used as the first reference current.

[0036] Here, the preset power factor, the first preset threshold, and the second preset threshold can be set according to actual needs, and this application does not limit them. Among them, when the preset power factor is 1, unity power factor is achieved, and the power transmission efficiency reaches its maximum.

[0037] In this embodiment, the three-phase isolated AC / DC converter includes a composite compensation unit 10 and an isolation conversion unit 20. The composite compensation unit 10 converts the three-phase AC power output from the three-phase AC power supply into DC power and controls the error between the average value of a first current and a first reference current to be less than a first preset threshold; wherein the first current is the current corresponding to the smallest absolute value of the current in the three-phase AC power. The isolation conversion unit 20 controls the error between the average value of its input current and a second reference current to be less than a second preset threshold, thereby indirectly controlling the current waveforms of the two phases with larger absolute current values ​​in the three-phase AC power, and thus controlling the output voltage and output power. This embodiment achieves the conversion between three-phase AC and DC power through the coordinated control of the composite compensation unit 10 and the isolation conversion unit 20, without relying on the bus capacitor for front-to-back decoupling. Therefore, there is no need to design a pre-charging circuit to pre-charge the bus capacitor, effectively improving the start-up response speed, simplifying the architecture of the three-phase isolated AC / DC converter, and reducing costs.

[0038] Based on the above embodiments, the following embodiment is provided to illustrate the structure of the composite compensation unit 10.

[0039] Figure 6 This is a schematic diagram of the structure of a three-phase isolated AC / DC converter provided in an embodiment of the present invention. Figure 2 In some embodiments, the composite compensation unit 10 can adopt the hardware structure of a two-level full-bridge PWM rectifier, which basically operates in power frequency mode and includes six switching transistors. For details, please refer to [reference needed]. Figure 6 As shown, the composite compensation unit 10 includes: a first inductor L1, a second inductor L2, a third inductor L3, and a three-phase bridge arm; Among them, the three-phase bridge arm includes the first bridge arm, the second bridge arm and the third bridge arm connected in parallel; The first end of the first inductor is connected to the first output terminal of the three-phase AC power supply, and the second end of the first inductor L1 is connected to the midpoint of the first bridge arm. The first end of the second inductor L2 is connected to the second output terminal of the three-phase AC power supply, and the second end of the second inductor L2 is connected to the midpoint of the second bridge arm. The first end of the third inductor L3 is connected to the third output terminal of the three-phase AC power supply, and the second end of the third inductor L3 is connected to the midpoint of the third bridge arm. The two ends of the first, second, and third bridge arms connected in parallel are respectively connected to the isolation conversion unit 20.

[0040] In some embodiments, the first bridge arm includes a first switch S1 and a second switch S2 connected in series, the second bridge arm includes a third switch S3 and a fourth switch S4 connected in series, and the third bridge arm includes a fifth switch S5 and a sixth switch S6 connected in series. The power frequency cycle of the three-phase AC power supply includes 6 sectors. For any sector, two target switching transistors in the three-phase bridge arm are in a high-frequency switching state so that the error between the average value of the first current and the first reference current is less than the first preset threshold. The target switching transistor is one of the two switching transistors in the three-phase bridge arm connected to the target inductor. The target inductor is one of the first inductor L1, the second inductor L2, and the third inductor L3, and the current flowing through the target inductor is the first current.

[0041] In one implementation scenario, the first switch S1 to the sixth switch S6 can be a field-effect transistor or an insulated gate bipolar transistor (IGBT).

[0042] Taking sector 1 as an example, the absolute value of the phase b current is the smallest at this time, so the phase b current is the first current. Since the first inductor L1 receives the phase a current, the second inductor L2 receives the phase b current, and the third inductor L3 receives the phase c current, the target inductor is the second inductor L2. Correspondingly, the target switching transistors are the third switching transistor S3 and the fourth switching transistor S4 in the second bridge arm of the three-phase bridge arm.

[0043] Here, controlling the high-frequency switching of the third switch S3 and the fourth switch S4 is used to make the error between the average value of the b-phase current and the first reference current less than the first preset threshold. Therefore, it is necessary to further determine the duty cycle of the third switch S3 and the fourth switch S4.

[0044] Figure 7 This is a schematic diagram of the structure of a first control unit provided in an embodiment of the present invention, with reference to... Figure 7 As shown, in some embodiments, the three-phase isolated AC / DC converter further includes a first control unit, which is used to determine the error between the first current and the first reference current, and to determine the duty cycle of the target switch based on the error between the first current and the first reference current, so that the error between the average value of the first current and the first reference current is less than a first preset threshold.

[0045] Taking sector 1 as an example, in one implementation scenario, the duty cycle D1 of the third switch S3 can be determined based on the error between the phase b current and the first reference current using a proportional-integral (PI) regulator or a proportional-integral-derivative (PID) regulator. The third switch S3 is then controlled based on its duty cycle Db. Furthermore, since the fourth switch S4 complements the third switch S3 in the high-frequency cycle, its duty cycle can also be determined, allowing for corresponding control of the fourth switch S4.

[0046] The above method achieves the goal of ensuring that the error between the average value of the first current and the first reference current is less than a first preset threshold through feedback. In addition, other methods such as feedforward, feedforward-feedback combination, or nonlinear control strategies can also be used to achieve this and improve steady-state and dynamic performance. This application does not limit the specific implementation method.

[0047] Except for the third switch S3 and the fourth switch S4, in sector 1, since Va>Vb>Vc, the second switch S2 and the fifth switch S5 and their body diodes are always in the off state in sector 1. As for the first switch S1 and the sixth switch S6, since the body diodes in the first switch S1 and the sixth switch S6 are conducting at this time, in order to reduce the loss caused by the voltage drop of the body diode, the first switch S1 and the sixth switch S6 can be controlled to always be in the conducting state in sector 1.

[0048] As can be seen from the above, within any sector, of the six switching transistors S1-S6 in the composite compensation unit 10, two switching transistors and their body diodes are permanently off, while the body diodes of two switching transistors are permanently on for uncontrolled rectification. The permanent on-state switching of these transistors is used for synchronous rectification to reduce conduction losses. Two target switching transistors are switched at high frequency to control the error between the average value of the first current and the first reference current to be less than a first preset threshold. Furthermore, since only two target switching transistors are switched at high frequency, the remaining four switching transistors operate at power frequency, resulting in lower losses.

[0049] Here, since the current of the target switching transistor under high-frequency switching conditions is the first current, that is, the current corresponding to the smallest absolute value of the current in the three-phase AC current, its maximum value is only half of the AC peak current, thus effectively reducing losses. At the same time, compared with the inductance of the power factor inductor in the front stage in the two-stage scheme, the inductance of the first inductor L1, the second inductor L2, and the third inductor L3 in the composite compensation unit 10 is also smaller.

[0050] Furthermore, the composite compensation unit 10 achieves a significant increase in the average input voltage and a substantial reduction in input voltage fluctuation of the isolation converter unit 20 at the cost of operating at high frequency only when the current is low. It can also utilize a more efficient but narrower gain range bidirectional excitation soft-switching topology such as LLC, significantly reducing the number of magnetic components and power semiconductor devices required in high-power applications. The specific structure of the isolation converter unit 20 can be found in the following embodiments, and will not be described in detail here.

[0051] Optionally, the composite compensation unit 10 may further include a sixth capacitor to absorb high-frequency noise, wherein the two ends of the parallel first, second, and third bridge arms are respectively connected to the sixth capacitor. Here, compared to the two-stage scheme, the capacitance required by the sixth capacitor is only tens or even hundreds of percent of the capacitance required by the two-stage scheme.

[0052] Figure 8a This is a schematic diagram comparing a first current and a first reference current provided in an embodiment of the present invention, wherein Icomp is the current of the target inductor, i.e., the first current, and IcompRef is the first reference current. Figure 8a It can be seen that the average value of Icomp is basically equal to the first reference current.

[0053] Figure 8b This is a waveform diagram of the drive signals of the switching transistors included in a composite compensation unit provided in an embodiment of the present invention. IL_a, IL_b, and IL_c are the currents of the three-phase AC power input to the composite compensation unit 10, respectively. GDS1-GDS6 are the waveforms corresponding to the drive signals of the switching transistors S1-S6, respectively. It can be seen that as the three-phase AC current changes, each switching transistor is either continuously on or continuously off for most of the time, and only operates at high frequency for 1 / 3 of the time. Moreover, when operating at high frequency, the absolute value of the current in that phase is the current with the smallest absolute value among the three-phase AC currents.

[0054] Here, we will use sector 1 as an example. Figure 9a This is a schematic diagram comparing the first current and the first reference current within sector 1 provided by an embodiment of the present invention. It can be seen that within sector 1, the average value of the first current Icomp is basically equal to the first reference current IcompRef.

[0055] Figure 9b This is a waveform diagram of the drive signals of the switching transistors included in the composite compensation unit 10 within sector 1, provided by an embodiment of the present invention. IL_a, IL_b, and IL_c are the current waveforms corresponding to the three-phase AC power input to the composite compensation unit, respectively, and GDS1-GDS6 are the waveforms corresponding to the drive signals of switching transistors S1-S6, respectively. Figure 9bIt can be seen that within sector 1, as the three-phase AC current changes, the drive signals of the first switch S1 and the sixth switch S6 are both high level, and the first switch S1 and the sixth switch S6 are in the conducting state; the drive signals of the second switch S2 and the fifth switch S5 are both low level, and the second switch S2 and the fifth switch S5 are in the off state; the drive signals of the third switch S3 and the fourth switch S4 are both high-frequency switching between high level and low level, and correspondingly, the third switch S3 and the fourth switch S4 are in a high-frequency switching state.

[0056] In this embodiment, the above Figure 6 The composite compensation unit 10 shown is a two-level structure. In another implementation scenario, the composite compensation unit 10 can also be an I-type three-level, T-type three-level, five-level, or seven-level structure, etc., and can achieve the same function according to the control strategy of this invention. For example, the T-type three-level structure can be referred to... Figure 10 As shown, Figure 10 This is a schematic diagram of a composite compensation unit with a T-type three-level structure provided in an embodiment of the present invention.

[0057] When the composite compensation unit 10 is adopted Figure 10 In the T-type three-level structure shown, if it is only used for rectification, the switching transistors S1-S6 can be replaced with diodes. By controlling the conduction and cutoff of S7-S12, the first current can be controlled. At this time, for sector 1, S1 and S6 are always in the conducting state, S2 and S5 are always in the cutoff state, and S3 and S4 are always in the high-frequency synchronous rectification state.

[0058] In another implementation scenario, the composite compensation unit 10 can also be connected in parallel with multiple channels, and the high-frequency ripple current can be reduced by interleaving high-frequency switches.

[0059] In summary, the composite compensation unit 10 converts three-phase AC power into pulsating DC power by controlling the switching of its included transistors, and also controls the first current. Furthermore, within any sector, only two of the six transistors in the composite compensation unit 10 operate at high frequency, and the current corresponding to this high-frequency condition is the current with the smallest absolute value among the three-phase AC currents. The other four transistors operate at the power frequency, thus effectively reducing switching losses. Additionally, the composite compensation unit 10 provided in this embodiment has a simple circuit, and given the rapid cost reduction of third-generation wide-bandgap power semiconductor devices such as SiC and GaN, it results in a more efficient, cost-effective, and compact three-phase isolated AC / DC converter.

[0060] Based on the above embodiments, the following embodiment is provided to illustrate the structure of the isolation transformation unit 20.

[0061] Still referencing Figure 6 As shown, the isolation conversion unit 20 includes a first bridge circuit, a first capacitor C1, a fourth inductor L4, a transformer T, a second bridge circuit, and a second capacitor C2. The first bridge circuit is used to convert the DC power output by the composite compensation unit 10 into AC power, and supply it to the primary winding of the transformer T through the first capacitor C1 and the fourth inductor L4; wherein, the first bridge circuit includes a fourth bridge arm and a fifth bridge arm connected in parallel, the midpoint of the fourth bridge arm is connected to the first end of the first capacitor C1, the second end of the first capacitor C1 is connected to the first end of the fourth inductor L4, the second end of the fourth inductor L4 is connected to the first end of the primary winding of the transformer T; the midpoint of the fifth bridge arm is connected to the second end of the primary winding of the transformer T; The second bridge circuit is used to receive the AC output from the secondary winding of transformer T and convert it into DC to supply the second capacitor C2 and the load; wherein, the second bridge circuit includes a sixth bridge arm and a seventh bridge arm connected in parallel, the first end of the secondary winding of transformer T is connected to the midpoint of the sixth bridge arm, and the second end of the secondary winding of transformer T is connected to the midpoint of the seventh bridge arm. The second capacitor C2 is connected in parallel with the second bridge circuit and the load for filtering.

[0062] In one implementation scenario, such as Figure 6 As shown, the fourth bridge arm includes the seventh switch S7 and the eighth switch S8 connected in series, the fifth bridge arm includes the ninth switch S9 and the tenth switch S10 connected in series, the sixth bridge arm includes the first diode D1 and the eleventh switch S11 connected in series, and the seventh bridge arm includes the second diode D2 and the twelfth switch S12 connected in series.

[0063] As can be seen from the above embodiments, the isolation converter 20 controls the output voltage and output power by controlling the error between the average value of its input current and the second reference current to be less than a second preset threshold. In one implementation scenario, the input current can be controlled by controlling the switching transistor in the isolation converter 20.

[0064] Figure 11 This is a schematic diagram of the structure of a second control unit provided in an embodiment of the present invention, with reference to... Figure 11 As shown, in some embodiments, the three-phase isolated AC / DC converter further includes a second control unit. The second control unit is used to determine the error between the input current of the isolated converter 20 and the second reference current, and to determine the control parameters of the isolated converter 20 based on the error between the input current of the isolated converter 20 and the second reference current, so that the error between the average value of the input current of the isolated converter 20 and the second reference current is less than a second preset threshold. The control parameters include at least one of the following: the duty cycle, switching frequency, and phase angle of the isolation conversion unit 20.

[0065] Figure 11 In this context, IIsoRef is the second reference current, and Ixz is the input current of the isolation converter 20. After determining the error between the second reference current and the input current of the isolation converter 20, the control parameters of the isolation converter 20 can be determined by a PI controller or a PID controller.

[0066] The control parameters include at least one of the duty cycle Dxz, switching frequency F, and phase angle φ of the isolation conversion unit 20, which can be specifically determined according to the topology of the isolation conversion unit 20.

[0067] In one implementation scenario, for the above Figure 6 The isolation converter 20 shown is a phase-shifted full-bridge topology with secondary-side switches, which can control the input current by adjusting the phase of the switching transistors S7-S10.

[0068] The above method achieves the goal of ensuring that the error between the average value of the isolation conversion unit 20 and the first reference current is less than the first preset threshold through feedback. In addition, other methods such as feedforward, feedforward-feedback combination, or nonlinear control strategy can also be used to achieve this and improve steady-state and dynamic performance. This application does not limit the specific implementation method.

[0069] Figure 12 This is a schematic diagram of the input current and output voltage of an isolation converter unit provided in an embodiment of the present invention. (Refer to...) Figure 12 As shown, Ixz is the input current of the isolation converter unit 20, IxzAvg is the average value of Ixz, IIsoRef is the second reference current, and Vo is the output voltage of the isolation converter unit 20. Figure 12 As can be seen, with the change of the three-phase AC current, i.e. under different sectors, the average value of the input current of the isolation converter unit 20, IxzAvg, remains basically consistent with the second reference current. At this time, the output voltage V0 of the isolation converter unit 20 only fluctuates slightly, and the three-phase AC currents Ia, Ib and Ic are all pure sine waves, thus achieving the purpose of realizing AC / DC energy transmission and controlling the AC input current.

[0070] remove Figure 6 Besides the phase-shifted full-bridge topology with secondary-side switches shown, the isolation converter 20 can also be selected from flyback, phase-shifted full-bridge, LLC, and other isolation DC / DC converter topologies in another implementation scenario. The structure of a three-phase isolated AC / DC converter using a multiphase interleaved flyback as the isolation converter 20 can be found in [reference needed]. Figure 13 As shown, the structure of a three-phase isolated AC / DC converter with LLC as the isolation conversion unit 20 can be referred to Figure 14 As shown.

[0071] For a multiphase interleaved flyback topology, the input power can be adjusted by changing the duty cycle of switches S7-Sn. For an LLC topology, the output power can be adjusted by changing the frequency of switches S7-S10.

[0072] In another implementation scenario, the isolation converter 20 can also adopt bidirectional isolated DC / DC topologies such as DAB, ACCL, and bidirectional flyback. This enables bidirectional energy flow from AC to DC. Only the phase of the three-phase target input current needs to be adjusted and distributed sector-wise to the composite compensation unit 10 and the isolation converter 20 as their reference current. The structure of a bidirectional AC / DC converter using DAB as the isolation converter 20 can be found in [reference needed]. Figure 15 As shown in the diagram. The three-phase target input current is determined based on the target output power, reactive power, and the voltage of the three-phase AC power. Here, the target output power can be considered as active power, and the corresponding power factor can be determined using the target output power and reactive power.

[0073] In this embodiment, the input voltage of the isolation converter unit 20 fluctuates only between 0.866 and 1 times the peak AC voltage, resulting in a significant increase in average voltage and a reduction in the effective value of the current in the circuit power components. Simultaneously, the smaller voltage fluctuation range allows for the selection of soft-switching topologies such as LLC as the isolation converter unit 20, which requires less resonant cavity circulating current, results in a smaller effective value of the current in the circuit components, and lower losses.

[0074] In summary, the isolation converter 20 adjusts at least one of the duty cycle, switching frequency, and phase angle of its included switching transistors to ensure that the error between the average value of the input current of the isolation converter 20 and the second reference current is less than a second preset threshold, thereby achieving control over the output voltage and output power. Furthermore, the isolation converter 20 also achieves isolated energy transfer through the transformer T. In addition, in this embodiment, the isolation converter 20 does not require a three-level structure, significantly simplifying the circuit and increasing the average input voltage, thus reducing the number of power components required for high-power applications.

[0075] Figure 16 This is a schematic diagram of the structure of a three-phase isolated AC / DC converter provided in an embodiment of the present invention. Figure 3 ,refer to Figure 16 As shown, optionally, the three-phase isolated AC / DC converter also includes a low-pass filter unit 30, which is disposed between the output terminal of the three-phase AC power supply and the composite compensation unit 10, to filter out the high-frequency ripple current generated by the composite compensation unit 10 and the isolation conversion unit 20, so as to reduce the adverse effects on the power grid.

[0076] Figure 17A schematic diagram comparing the three-phase AC current waveforms at the input and output ports of a low-pass filter unit provided in this application embodiment is shown below. Figure 17 As shown, Grid_Ia, Grid_Ib, and Grid_Ic are the current waveforms corresponding to the three-phase AC current at the input port of the low-pass filter unit 30, and IL_a, IL_b, and IL_c are the current waveforms corresponding to the three-phase AC current at the output port of the low-pass filter unit 30. It can be seen that the low-pass filter unit 30 can effectively reduce high-frequency ripple current.

[0077] In one implementation scenario, a second-order LC low-pass filter can be selected as the low-pass filter unit 30. For specific details, please refer to [reference needed]. Figure 16 As shown, the low-pass filter unit 30 includes a fifth inductor L5, a sixth inductor L6, a seventh inductor L7, a third capacitor C3, a fourth capacitor C4, and a fifth capacitor C5. Among them, the first end of the fifth inductor L5 is connected to the first output terminal of the three-phase AC power supply, and the second end is connected to the first end of the first inductor L1; the first end of the sixth inductor L6 is connected to the second output terminal of the three-phase AC power supply, and the second end is connected to the first end of the second inductor L2; the first end of the seventh inductor L7 is connected to the third output terminal of the three-phase AC power supply, and the second end is connected to the first end of the third inductor L3. The first terminal of the third capacitor C3 is connected between the second terminal of the fifth inductor L5 and the first terminal of the first inductor L1. The first terminal of the fourth capacitor C4 is connected between the second terminal of the sixth inductor L6 and the first terminal of the second inductor L2. The first terminal of the fifth capacitor C5 is connected between the second terminal of the seventh inductor L7 and the first terminal of the third inductor L3. The second terminals of the third capacitor C3, the fourth capacitor C4, and the fifth capacitor C5 are interconnected.

[0078] The fifth inductor L5, the sixth inductor L6, the seventh inductor L7, the third capacitor C3, the fourth capacitor C4, and the fifth capacitor C5 mentioned above are all used for filtering.

[0079] In another implementation scenario, if the composite compensation unit 10 and the isolation conversion unit 20 have reduced the high-frequency ripple current to an acceptable range by means of interleaving in parallel and increasing the frequency, then the low-pass filter unit 30 can be simplified or omitted.

[0080] The low-pass filter unit 30 can be simplified to have only inductors, only capacitors, or a Π-type filter network composed of inductors and capacitors. Since the low-pass filter unit 30 often reuses circuit devices with the electromagnetic compatibility unit (EMC) of a three-phase AC system, when the composite compensation unit 10 and the isolation conversion unit 20 are composed of multiple units connected in parallel in an interleaved manner, its function can be replaced by the EMC when the low-pass filter unit 30 is omitted.

[0081] In summary, a low-pass filter unit 30 is installed between the output terminal of the three-phase AC power supply and the composite compensation unit 10 to eliminate the high-frequency ripple current generated by the composite compensation unit 10 and the isolation conversion unit 20, thereby reducing the adverse effects on the power grid.

[0082] Figure 18 This is a flowchart illustrating a control method for a three-phase isolated AC / DC converter provided in an embodiment of the present invention, applied to any of the three-phase isolated AC / DC converters provided in the above embodiments, including: S1801: Obtain the target output power and the voltage of the three-phase AC power output from the three-phase AC power supply.

[0083] Here, the target output power is related to the load, and different loads correspond to different target output powers.

[0084] In one implementation scenario, the target output power can be set by the user.

[0085] In another implementation scenario, for charging new energy vehicles, the vehicle's battery management system can send a corresponding command carrying the target output power. This application does not limit the method for determining the target output power.

[0086] In one implementation scenario, the three-phase AC power output from the three-phase AC power supply can be three-phase mains power, and its current and voltage waveforms can be referenced above. Figure 5 As shown.

[0087] S1802: Determine the first reference current and the second reference current based on the voltage of the three-phase AC power, the target output power, and the preset power factor.

[0088] In one implementation scenario, determining the first reference current and the second reference current based on the voltage of the three-phase AC power, the target output power, and the preset power factor includes: determining the sector corresponding to the three-phase AC power supply at the current moment based on the current voltage of the three-phase AC power; determining the three-phase target input current based on the voltage of the three-phase AC power, the target output power, and the preset power factor; and determining the first reference current and the second reference current corresponding to the current sector based on the three-phase target input current.

[0089] Let the current voltage of the three-phase AC power be: Va = Vm Cos(ωt) Vb = Vm Cos(ωt-120°) Vc = Vm Cos(ωt-240°) Where Va, Vb, and Vc are the voltages of the three-phase alternating current, Vm represents the peak value of the voltage, w represents the phase, and t represents the time.

[0090] As can be seen from the above embodiments, the power frequency cycle of a three-phase AC power supply can be divided into 6 sectors. Specifically, when Va>Vb>Vc, it corresponds to sector 1; when Vb>Va>Vc, it corresponds to sector 2; when Vb>Vc>Va, it corresponds to sector 3; when Vc>Vb>Va, it corresponds to sector 4; when Vc>Va>Vb, it corresponds to sector 5; and when Va>Vc>Vb, it corresponds to sector 6.

[0091] Once the voltage of the three-phase AC power is acquired at any given moment, the corresponding sector can be determined based on the magnitude of the three-phase AC power voltage.

[0092] In one implementation scenario, if the target output power is Po, and the preset power factor is 1 (i.e., unity power factor), then the three-phase target input currents are as follows: Ia = (Po / (3Vm))Cos(ωt) Ib = (Po / (3Vm))Cos(ωt-120°) Ic = (Po / (3Vm))Cos(ωt-240°) Where Ia, Ib, and Ic are the three-phase target input currents, Po is the target output power, Vm represents the peak voltage, w represents the phase, and t represents the time.

[0093] As can be seen from the above embodiments, the composite compensation unit is used to control the error between the average value of the first current and the first reference current to be less than a first preset threshold. The first current is the current corresponding to the smallest absolute value of the current in the three-phase AC power. Therefore, the target input current corresponding to the smallest absolute value of the current in the three-phase AC power can be used as the first reference current.

[0094] For example, for sector 1, the voltage relationship of the three-phase AC is Va>Vb>Vc. At this time, the absolute value of the current in phase b is the smallest. Therefore, the first reference current ICompRef can be set as the target input current corresponding to phase b, i.e. (Po / (3Vm))Cos(ωt-120°).

[0095] Here, the second reference current corresponding to the input current of the isolation converter unit can be determined by the target output power, the first reference current, and the voltage of the three-phase AC power. For sector 1, the second reference current can be set as follows:

[0096] Where IIsoRef represents the second reference current, Po is the target output power, w represents the phase, and t represents the time.

[0097] It should be noted that the embodiments in this application are only illustrated by sector 1. The control principles of other sectors are the same as those of sector 1, and will not be described in detail here.

[0098] S1803: Control the composite compensation unit according to the first reference current to convert the three-phase AC power output from the three-phase AC power supply into DC power, and make the error between the average value of the first current and the first reference current less than the first preset threshold; wherein, the first current is the current corresponding to the smallest absolute value of the current in the three-phase AC power.

[0099] In one implementation scenario, the first current can be acquired in real time, and the error between the first current and the first reference current can be further determined. Based on this error, the duty cycle of the switching transistors included in the composite compensation unit can be determined by a PI regulator or PID regulator, thereby adjusting the magnitude of the first current so that the error between the average value of the first current and the first reference current is less than a first preset threshold.

[0100] S1804: Control the isolation conversion unit according to the second reference current so that the error between the average value of the input current of the isolation conversion unit and the second reference current is less than the second preset threshold.

[0101] The input current of the isolation converter is the DC output of the composite compensation unit. By controlling the error between the average value of the input current of the isolation converter and the second reference current to be less than a second preset threshold, the current waveforms of the two phases with larger absolute current values ​​in the three-phase AC power can be indirectly controlled, thereby achieving control of the output voltage and output power.

[0102] In one implementation scenario, the input current of the isolation unit can be acquired in real time, the error between the input current and the second reference current can be determined, and further, based on this error, the control parameters of the isolation conversion unit can be determined by a PI regulator or PID regulator, and the magnitude of the input current of the isolation conversion unit can be adjusted so that the error between the average value of the input current of the isolation conversion unit and the second reference current is less than a second preset threshold.

[0103] The control parameters include at least one of the duty cycle, switching frequency, and phase angle of the switching transistors included in the isolation conversion unit.

[0104] This embodiment provides a control method for a three-phase isolated AC / DC converter. After obtaining the target output power and the voltage of the three-phase AC power output, a first reference current and a second reference current are determined based on the target output power, the voltage of the three-phase AC power, and a preset power factor. Furthermore, the composite compensation unit is controlled based on the first reference current to convert the three-phase AC power into DC power, ensuring that the error between the average value of the first current and the first reference current is less than a first preset threshold. The isolation conversion unit is controlled based on the second reference current to ensure that the error between the average value of the input current of the isolation conversion unit and the second reference current is less than a second preset threshold. This indirectly controls the current waveforms of the two phases with larger absolute current values ​​in the three-phase AC power, thereby controlling the output voltage and output power. This embodiment achieves the conversion of three-phase AC power to DC power through the coordinated control of the composite compensation unit and the isolation conversion unit. It does not rely on the bus capacitor for decoupling between the front and rear stages, thus eliminating the need for a pre-charging circuit to pre-charge the bus capacitor, effectively improving the start-up response speed, simplifying the architecture of the three-phase isolated AC / DC converter, and reducing costs.

[0105] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0106] In the above embodiments, the descriptions of each embodiment have their own emphasis. Parts not detailed or described in a particular embodiment can be referred to in the relevant descriptions of other embodiments. Unless otherwise specified or in conflict with logic, the terminology and / or descriptions between different embodiments are consistent and can be referenced interchangeably. Technical features in different embodiments can be combined to form new embodiments based on their inherent logical relationships.

[0107] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A three-phase isolated AC / DC converter, characterized in that, include: A composite compensation unit is used to convert the three-phase AC power output from the three-phase AC power supply into DC power, and to control the error between the average value of the first current and the first reference current to be less than a first preset threshold; wherein, the first current is the current corresponding to the smallest absolute value of the current in the three-phase AC power. An isolation conversion unit, connected to the composite compensation unit, is used to control the error between the average value of the input current of the isolation conversion unit and the second reference current to be less than a second preset threshold. The first reference current and the second reference current are determined based on the target output power and the preset power factor.

2. The three-phase isolated AC / DC converter according to claim 1, characterized in that, The composite compensation unit includes: a first inductor, a second inductor, a third inductor, and a three-phase bridge arm; The three-phase bridge arm includes a first bridge arm, a second bridge arm, and a third bridge arm connected in parallel; The first end of the first inductor is connected to the first output terminal of the three-phase AC power supply, and the second end of the first inductor is connected to the midpoint of the first bridge arm. The first end of the second inductor is connected to the second output terminal of the three-phase AC power supply, and the second end of the second inductor is connected to the midpoint of the second bridge arm. The first end of the third inductor is connected to the third output terminal of the three-phase AC power supply, and the second end of the third inductor is connected to the midpoint of the third bridge arm. The two ends of the first, second, and third bridge arms connected in parallel are respectively connected to the isolation conversion unit.

3. The three-phase isolated AC / DC converter according to claim 2, characterized in that, The first bridge arm includes a first switch and a second switch connected in series; the second bridge arm includes a third switch and a fourth switch connected in series; and the third bridge arm includes a fifth switch and a sixth switch connected in series. The power frequency cycle of the three-phase AC power supply includes 6 sectors. For any sector, two target switching transistors in the three-phase bridge arm are in a high-frequency switching state so that the error between the average value of the first current and the first reference current is less than a first preset threshold. The target switching transistors are two switching transistors in the bridge arm connected to the target inductor in the three-phase bridge arm. The target inductor is one of the first inductor, the second inductor, and the third inductor, and the current flowing through the target inductor is the first current.

4. The three-phase isolated AC / DC converter according to claim 3, characterized in that, The three-phase isolated AC / DC converter further includes a first control unit, which is used to determine the error between the first current and the first reference current, and to determine the duty cycle of the target switch based on the error between the first current and the first reference current, so that the error between the average value of the first current and the first reference current is less than the first preset threshold.

5. The three-phase isolated AC / DC converter according to claim 1 or 2, characterized in that, The isolation conversion unit includes a first bridge circuit, a first capacitor, a fourth inductor, a transformer, a second bridge circuit, and a second capacitor. The first bridge circuit is used to convert the DC power output by the composite compensation unit into AC power, and provide it to the primary winding of the transformer through the first capacitor and the fourth inductor; wherein, the first bridge circuit includes a fourth bridge arm and a fifth bridge arm connected in parallel, the midpoint of the fourth bridge arm is connected to the first end of the first capacitor, the second end of the first capacitor is connected to the first end of the fourth inductor, the second end of the fourth inductor is connected to the first end of the primary winding of the transformer; the midpoint of the fifth bridge arm is connected to the second end of the primary winding of the transformer; The second bridge circuit is used to receive the alternating current output from the secondary winding of the transformer and convert it into direct current to supply the second capacitor and the load; wherein, the second bridge circuit includes a sixth bridge arm and a seventh bridge arm connected in parallel, the first end of the secondary winding of the transformer is connected to the midpoint of the sixth bridge arm, and the second end of the secondary winding of the transformer is connected to the midpoint of the seventh bridge arm. The second capacitor is connected in parallel with the second bridge circuit and the load, respectively, for filtering.

6. The three-phase isolated AC / DC converter according to claim 5, characterized in that, The three-phase isolated AC / DC converter further includes a second control unit, which is used to determine the error between the input current of the isolated converter and the second reference current, and to determine the control parameters of the isolated converter based on the error between the input current of the isolated converter and the second reference current, so that the error between the average value of the input current of the isolated converter and the second reference current is less than the second preset threshold. The control parameters include at least one of the following: the duty cycle, switching frequency, and phase angle of the isolation conversion unit.

7. The three-phase isolated AC / DC converter according to any one of claims 2-6, characterized in that, The three-phase isolated AC / DC converter also includes a low-pass filter unit, which is disposed between the output terminal of the three-phase AC power supply and the composite compensation unit, for filtering out the high-frequency ripple current generated by the composite compensation unit and the isolated conversion unit.

8. The three-phase isolated AC / DC converter according to claim 7, characterized in that, The low-pass filter unit includes a fifth inductor, a sixth inductor, a seventh inductor, a third capacitor, a fourth capacitor, and a fifth capacitor; Wherein, the first end of the fifth inductor is connected to the first output terminal of the three-phase AC power supply, and the second end is connected to the first end of the first inductor; the first end of the sixth inductor is connected to the second output terminal of the three-phase AC power supply, and the second end is connected to the first end of the second inductor; the first end of the seventh inductor is connected to the third output terminal of the three-phase AC power supply, and the second end is connected to the first end of the third inductor. The first end of the third capacitor is connected between the second end of the fifth inductor and the first end of the first inductor; the first end of the fourth capacitor is connected between the second end of the sixth inductor and the first end of the second inductor; the first end of the fifth capacitor is connected between the second end of the seventh inductor and the first end of the third inductor; and the second ends of the third capacitor, the fourth capacitor, and the fifth capacitor are interconnected.

9. A control method for a three-phase isolated AC / DC converter, applied to the three-phase isolated AC / DC converter according to any one of claims 1-8, characterized in that, include: Obtain the target output power and the voltage of the three-phase AC power output from the three-phase AC power supply; The first reference current and the second reference current are determined based on the voltage of the three-phase AC power, the target output power, and the preset power factor. The composite compensation unit is controlled according to the first reference current to convert the three-phase AC power output from the three-phase AC power supply into DC power, and to make the error between the average value of the first current and the first reference current less than a first preset threshold; wherein, the first current is the current corresponding to the smallest absolute value of the current in the three-phase AC power. The isolation conversion unit is controlled according to the second reference current so that the error between the average value of the input current of the isolation conversion unit and the second reference current is less than a second preset threshold.

10. The control method for a three-phase isolated AC / DC converter according to claim 9, characterized in that, The step of determining the first reference current and the second reference current based on the voltage of the three-phase AC power, the target output power, and the preset power factor includes: Based on the current voltage of the three-phase AC power supply, determine the sector corresponding to the three-phase AC power supply at the current moment; The target three-phase input current is determined based on the voltage of the three-phase AC power, the target output power, and the preset power factor. Based on the three-phase target input current, determine the first reference current and the second reference current corresponding to the current sector.

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